The present invention relates generally to integrated circuit thin film resistors, and more particularly to a thin film resistor structure and method that provide very low variance in the head resistivity of thin film resistors.
FIG. 1 shows a plan view of a prior art thin film resistor 30, which may be composed of, for example, sichrome (SiCr), designated by reference numeral 2. FIG. 2 shows a section view of thin film resistor structure 30 of FIG. 1. Referring to FIGS. 1 and 2, thin film resistive layer 2 of resistor structure 30 has an elongated rectangular configuration of width W, including a central body portion of length L and two identical “head” sections 22A and 22B contacted by metal conductors 24A and 24B, respectively.
FIG. 2 shows an accurate section view of a portion of a prior art integrated circuit including a prior art thin film resistor structure in which a sichrome (SiCr) thin film resistor 2 formed on a layer or region 18 which includes several conventional “pre-metal” dielectric layers (not shown). The term “pre-metal dielectrics” is well-known in the integrated circuit industry, and includes different contiguous pre-metal dielectric layers having somewhat different doping, including, for example, for example one or more, boron-phosphorus “TEOS” (tetrethylorthosilicate) layers.
The pre-metal dielectric layer 18 may be, but typically is not chemically/mechanically polished (CMP) to precisely planarize the top surface thereof before a next layer is applied to the integrated circuit. Accordingly, the upper surface of dielectric layer 18 actually may be very smooth or may be quite “rough”. Layer 18 is formed on a silicon layer 16, which itself may be formed on a silicon wafer substrate.
SiCr thin film resistor is 2 formed on an “interlevel dielectrics” layer or region 21 formed of several dielectric sub-layers (not shown). Thin film resistor “head” 22A may be composed of TiW (titanium—tungsten) which extends through an opening in a dielectric sub-layer of oxide region 20 to make electrical contact with the left end or head of SiCr resistor 2, and also makes contact with a portion 24A of a metallization layer 24A,B, typically formed of aluminum, formed on the upper surface of interlevel dielectric region 21. In a similar manner, a separate portion 24B of metallization layer 24A,B makes electrical contact to the right end or head of SiCr resistor 2. Interconnect “Metal 2” conductors 24A and 24B extend along the surface of dielectric layer 21 and are connected to electrodes of various circuit elements (not shown) such as transistors, capacitors, and resistors, and may also be connected by appropriate conductive vias to “Metal 1” conductors such as conductor 9.
FIG. 3 shows a graph of the product of RTOTAL and WCORRECTED for various resistor body lengths L. RTOTAL is the total resistance of a thin film resistor of a particular width, and WCORRECTED is equal to W−ΔW, where ΔW is the error in resistor width W that occurs during the manufacturing processes. The slope of a line through measured data points 35 is equal to the sheet resistance RSBODY of the thin film resistor.
The equation for the resistance of resistor 30 of FIG. 1 isRTOTAL=RSBODY*[L/(W−ΔW)]+2RHEAD,where RSBODY is the sheet resistance of the body of the resistor and RHEAD is the head resistance of one end section or head section of the thin film resistor 30.
The head resistance RHEAD is given by the equationRHEAD=[RHEADRSTVTY*LHEAD]/AHEAD,where RHEADRSTVTY is the head resistivity, LHEAD is the length of the resistor head, and AHEAD is the area of the resistor head. AHEAD is equal to LHEAD*(W−ΔW). The head resistivity RHEADRSTVTY is equal to the head resistance RHEAD multiplied by the corrected thin film resistor width W−ΔW. From this, the following equation can be written for RTOTAL:RTOTAL=RSBODY*[L/(W−ΔW)]+2*RHEADRSTVTY/(W−ΔW).From this, the equation for a line through the data points 35 shown in FIG. 3 can be written as:RTOTAL*(W−ΔW)=RSBODY*LBODY+2*RHEADRSTVTY.Note that there are other ways of determining the head resistivity RHEADRSTVTY. For example, three different thin-film resistors could be used, two of the same width and two of the same length, to determine a ΔW, a ΔL and a sheet resistance RSBODY by simultaneous solution of three equations. With three resistances being measured and the lengths L and widths W being known quantities for the three resistors, the three equations can be solved simultaneously to obtain the values of ΔL, ΔW and the sheet resistance. Three such equations for three resistors R1 of length L1 and width W1, R2 of length L2 and width W2, and R3 of length L3 and width W3 are:ΔL=((R1*L3)−(R3*L1))/(R3−R1), assuming W3=W1,ΔW=((R2*W2)−(R3*W3))/(R3−R2), assuming L2=L3, andRSBODY=R2((W2+ΔW)/(L2+ΔL)).The head resistivity is related to the ΔL term, so if the resistance is high it is treated as if the resistor got longer. If the resistance is low is treated as if the resistor got shorter. In either case, the head resistivity is included as part of the ΔL term.
It should be appreciated that increasing the precision of the thin film resistors as much as possible requires minimizing the variance of the head resistivity. The above equation for the total resistance RTOTAL of resistor 2 includes terms for the two head resistivities and the length and width of the resistor body, and that the present invention focuses on minimizing/reducing the variance of the head resistivity component of the resistor.
It also should be appreciated that for very low value thin film resistors, i.e., very short thin film resistors, for example in a string DAC (digital to analog converter), where the string resistors may have low values of, for example, about 140 ohms to as much as 1000 ohms, the head resistivity may be a very large proportion of the total resistance of a particular resistor. Consequently, obtaining high accuracy in manufacture of the string DACs requires increasing the accuracy of the resistance of each resistor in the string DAC, and that requires controlling the integrated circuit manufacturing process so as to minimize the variance of the head resistivities of the thin-film string resistors.
The actual or “corrected” thin film resistor width W−ΔW is the difference between the design value W and an error amount ΔW. Typically, thin-film resistors experience a width change ΔW during manufacture which in some cases it may be very significant, and the value ΔW is the difference between the design value of W and the actual width of the resistor after it is fabricated. (Usually, after ΔW becomes known, the appropriate reticle of the mask set then is corrected by that amount ΔW so the actual width of the resistors in subsequently manufactured integrated circuits is very precisely equal to W.)
Head resistivity can be determined empirically by manufacturing a number of thin film resistors that are of the same width but all the various lengths. The ΔW is expected to be the same for all of them, so a graph can be plotted as shown in FIG. 3 and the head resistivity can be determined as the “Y intercept” of that data.
Thus, head resistivity of a thin film resistor can be accurately determined for a sufficient number of thin film resistors manufactured by a particular process and the accuracy and variance of the head resistivity for the process can be determined.
“Dummy fill” has been frequently utilized in conjunction with use of chemical/mechanical polishing (CMP) in integrated circuit chip fabrication processes. Dummy fill also has been utilized beneath an array of thin film resistors to disperse laser beam energy reflected during trimming of the thin film resistors so as to reduce optical interference of reflected laser energy with the incident laser beam to thereby improve laser trimming of the thin film resistors, as described in commonly assigned U.S. Pat. No. 6,818,966 issued Nov. 16, 2004 to Beach et al. In CMP processes, it is necessary to have an adequate density of the materials being polished to avoid localized over-polishing, referred to as “dishing”, which results in a non-planar surface after the polishing. Non-planar surfaces are incompatible with many conventional integrated circuit processing steps. For example, if interconnect metallization is deposited on a non-planar surface and the resulting surface then is subjected to a CMP operation, there may be residues of undesired metallization which are not adequately removed. Such undesired metallization residues may cause electrical shorting or other problems that lower integrated circuit processing yield.
When CMP operations are performed in the vicinity of thin film resistors, the “dishing” referred to above may cause large “systematic” errors in the resistances and ratio-matching of resistances of the thin film resistors. This is because stress in the thin film resistive material, especially SiCr which is somewhat piezo-resistive, causes identical SiCr resistor segments to have slightly different resistances, due in part to the variation in piezo-resistivity. Not only is the resistance of every identical resistor or resistor segment different for different die and different wafers, the systematic error associated with the resistor segments typically varies significantly even within the same die.
For prior art integrated circuit surface planarizing processes, it was considered unacceptable to have any metal or any other abrupt integrated circuit topology features located underneath thin film resistors. This was because the prior art integrated circuit surface planarizing processes could not adequately planarize or precisely flatten such topology features well enough to avoid severe disruption of the matching and stability of the thin film resistors due to material stresses and/or discontinuities (especially in very thin layers such as SiCr which, for example, may be only about 30 Angstroms thick) and/or optical inaccuracies associated with photolithography techniques being used.
Thus, there is an unmet need for an improved, inexpensive integrated circuit thin-film resistor structure and method for reducing or eliminating inaccuracy in the resistances and ratios of resistances of thin-film resistors.
There also is an unmet need for an improved, inexpensive integrated thin-film resistor structure and method for reducing variance in the head resistivity of thin-film resistors.